The statements in this section merely provide background information related to the present disclosure and may not constitute prior art.
Diesel engines are operated at higher than stoichiometric air-to-fuel mass ratios for improved fuel economy. Such lean-burning engines produce a hot exhaust with a relatively high content of oxygen and nitrogen oxides (NOx). The temperature of the exhaust from a warmed-up diesel engine is typically in the range of about 200° C. to about 400° C. and has a representative composition, by volume, of about 10% to about 17% oxygen, about 3% carbon dioxide, about 0.1% carbon monoxide, about 180 parts per million (ppm) hydrocarbons, about 235 ppm NOx and the balance nitrogen and water.
The NOx gases, typically comprising nitric oxide (NO) and nitrogen dioxide (NO2) are difficult to reduce to nitrogen (N2) because of the high oxygen (O2) content in the hot exhaust stream. NOx reduction technologies in automotive lean-burn engine exhausts include selective catalytic reduction by urea (urea/SCR) and lean NOx trap (LNT) technologies. However, these technologies have limitations for their implementation in vehicles. The urea/SCR technology requires an infrastructure for urea distribution, storage and delivery, while the LNT technology requires a sophisticated engine control technology and frequent regeneration of catalysts due to sulfur poisoning.
General Motors Corporation has developed a hydrocarbon plus oxygenated hydrocarbon/selective catalytic reduction (HC+OHC)/SCR technology that uses onboard diesel fuel as the source of the NOx reductant. For example, U.S. Patent Publication 20080127634, published Jun. 5, 2008, corresponding to application Ser. No. 11/566,693, the disclosure of which is incorporated herein by reference, describes, in embodiments, a method for reducing nitrogen oxides including NO and NO2 in an exhaust stream also comprising oxygen, carbon monoxide and hydrocarbons at a temperature above about 150° C., the method including, in embodiments: oxidizing NO in the exhaust stream to NO2; reforming raw diesel fuel to produce OHC's from the fuel; adding diesel fuel hydrocarbons and their oxygenates to the exhaust stream for the reduction of nitrogen oxides; and contacting the exhaust stream with a hybrid reduction catalyst comprising BaY—Ag/Al2O3 to reduce the nitrogen oxides to N2. The method reduces NOx initially through the (HC+OHC)/SCR process producing N2, while also producing NH3 as a byproduct over Ag/Al2O3. The NH3 subsequently reduces NOx over the BaY—Ag/Al2O3 hybrid catalyst through the NH3/SCR process, producing more N2 and thereby enhancing the NOx reduction efficiency of the hybrid catalyst system.
The appropriate components and process aspects of each of the foregoing U.S. Patents and Applications may be selected for the present disclosure in embodiments thereof.